The telecommunication industry provides customers with constant, around the clock access to communication services. Downtime is not permitted or tolerated in this industry. As such, most communication systems require some form of redundancy for all or most of the underlying equipment and components. The actual power supply and back-up power supply is no exception. Thus, many telecom and/or networking equipment are required to operate from a dual back-up battery or power supply (as referred to as a “plant”).
An example of a prior art telecommunication device 10 is shown in FIG. 1. Telecommunication device 10 includes a network element 12 connected to dual back-up batteries 14a, 14b (collectively referenced as dual batteries 14). The dual batteries 14 are connected in a parallel circuit configuration, with each input feeding into one of four “OR-ing” diodes 16a, 16b, 16c and 16d (collectively referenced as OR-ing diodes 16). The OR-ing diodes 16 conduct current in the forward direction and prevent the flow of return current back through the dual batteries 14. Current flows through the circuit in the direction indicated by arrows in FIG. 1. Current flowing from the positive terminals is indicated by solid arrowheads, while current flowing into the negative terminals is indicated as a line arrowhead.
The dual batteries 14 supply power to DC/DC converters 18 and transfer this energy to other components (not shown) of the network element 12. The dual batteries 16 are connected to the building ground 20 via a “vertical riser” 22. The vertical riser 22 is generally a cable, around 750 cm in length, connecting the positive terminals of each battery 14 to each other, to building ground 20 and to the chassis ground 24 of the network element 12. When both batteries are connected and working properly, i.e., at virtually the same voltage level, both the vertical riser 22 and the chassis ground 20, ideally, should carry virtually no current due to a balanced distribution of current flow in all feeds and returns. In other words, the currents flowing through the vertical riser 22 from the positive terminals of the batteries 14 are of equal magnitude and in opposite directions, thereby, effectively cancelling each other out and allowing no current to flow. Minor imbalances, if any, are due to slight resistance deltas in feeders, junctions, taps and the voltage and/or current properties of diodes.
Although the current load through ground is negated when both batteries 14 are connected and working properly, problems arise when one battery becomes disconnected or suffers a capacity/stored voltage loss. For example, as shown in the block diagram of FIG. 2, if the negative terminal of battery 14a is disconnected, then no current flows from the positive terminal of battery 14a to offset the current from the positive terminal of battery 14b. Thus, a direct path to building ground is established from the positive terminal of battery 14b through the vertical riser 22. The vertical riser 22 current may be up to half of the total plant current, if all network elements are powered off the same set of dual back-up batteries, and may exceed the rating of the vertical riser cable. The vertical riser 22 is generally not rated high enough to support this excess current and can potentially cause overheating of the cable. A similar problem is also created when the batteries 14 are not at the same voltage level, reverse biasing (i.e., disconnecting) one of the OR-ing diodes 16 causing excessive current to flow into the vertical riser 22.
Therefore, what is needed is a battery return disconnect switch for network elements powered by a dual battery supply that prevents an excess current from affecting the building ground.